Our preliminary studies and those of others clearly demonstrate a dominance of a single species of spotted fever group rickettsiae in ixodid tick vectors, Dermacentor variabilis and D. andersoni. Extensive tick surveys in Montana, Ohio, California, and Maryland revealed the dominance of nonpathogenic rickettsiae in tick populations. The presence of nonpathogenic tick symbionts may account for the low infection rates of Rickettsia rickettsii in D. andersoni and D. variabilis thus resulting in patchy distribution of human cases of Rocky Mountain spotted fever in the US. This project was initiated to investigate the molecular consequences and epidemiological significance of multiple rickettsial infections in ticks. We have proposed that the interactive effects between obligate intracellular rickettsiae within individual ticks may be of sufficient relevance and magnitude to alter the vector competence of ticks. Thus, we have focused on rickettsial symbionts acquired transovarially by D. variabilis ticks, namely Rickettsia montanensis and R. peacockii. We have investigated the prevalence and relative efficiency of transovarial interference of R. montanensis within D. variabilis ticks as mediated by R. rhipicephali. We have also focused on the D. variabilis genes that are differentially expressed in response to rickettsial infection within tick ovarian tissues. We have constructed several tissue-specific cDNA libraries from infected and uninfected D. variabilis. For the renewal application, we are proposing to further refine and characterize molecular mechanisms underlying transovarial interference. Studies under first aim define the functional role of identified molecules that initiate the exclusion of secondary rickettsial infections of tick ovaries. The second aim will be focused on investigating the role of the selected tick molecules in the maintenance and transmission of rickettsiae. [unreadable] [unreadable]